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Move to Li-ion requires many technical changes

14-Sep-2009 05:14 EDT

Ford is devising a range of control techniques for the lithium-ion batteries that are the mainstay of its electrification program.

Battery technologies recently got a huge shot in the arm when federal grants were handed out. This portion of the economic stimulus package aims to make lithium-ion the technical base for electrified vehicles, but it also underscores the many technical changes automakers and their suppliers must make.

Battery companies got the bulk of $2.4 billion in grant monies given to transportation companies by the Department of Energy in August. To name a few, A123Systems got $249 million, BASF collected $24.6 million, EnerDel Inc. received $118.5 million, and Johnson Controls was given $299 million. Many others got funding designed to build an infrastructure for hybrid and electric drive vehicles.

The moves mark a significant advance in the move to electrified vehicles, but the transition also means that engineers will have to make a number of changes in their control systems. Li-ion brings significant benefits over the NiMH batteries now used in hybrids.

Foremost among them is the ability to deliver more power from smaller batteries. Most automakers will leverage that to shrink the size of battery packs.

“The nice thing about lithium is that you can get smaller batteries,” said Larry Nitz, General Motors Executive Director for Hybrid Powertrains. “We won’t use lithium to get more power.”

With lithium, mass can be roughly cut in half. That saves some space, but more significantly it reduces weight. For example, the NiMH battery pack in the Ford Fusion weighs around 60 kg (132 lb), with cells accounting for about 40 kg (88 lb).

“The cell weight of an equivalent lithium system will be around 20 kg," said Ted Miller, Senior Manager of Energy Storage Strategy at Ford. The transition to lithium is planned throughout the industry. For example, Ford plans to use Li-ion in its electric Transit Connect van, set for introduction next year.

The transition to lithium-ion batteries will not depend solely on production capability. There are a number of technical issues that must be addressed to make the transition from NiMH. The NiMH battery packs cannot be managed by the control techniques in use today.

“We would like to establish the same basic control structure for both nickel metal-hydride and lithium-ion batteries," said Toshifumi Takaoka, General Manager of Toyota’s Hybrid Vehicle Engineering Management Division. “However, the internal resistance and other characteristics of these batteries differ widely, and we think that these areas will have to be controlled using separate modules.”

A key factor is that Li-ion batteries are delicate and are heavily dependent on the control module to ensure that battery lifetimes match powertrain life cycles.

“Lithium-ion batteries will quickly degrade when charged to 100% full capacity or discharged to 0% state-of-charge,” said Greg Zimmer, Product Marketing Engineer for Signal Conditioning Products at Linear Technology Corp. “Only a limited range of each cell’s full capacity can be used if the entire battery pack is to have a long life.”

Creating a management system to maintain lifetimes and gain the size benefits is significantly more complex than controlling NiMH battery packs. Engineers need to look at more parameters in order to match battery pack lifetimes with expectations for the vehicle itself.

“NiMH battery packs are typically self leveling, so one side does not get out of balance,” said Jim Harkins, Senior Manager of Denso International America's Engineer Electrical Engineering department. “Li-ion doesn’t have self leveling, so it’s more critical to maintain temperatures, control leveling, and employ cell balancing to maintain lifetimes.”

When a battery is repeatedly charged and discharged, its internal resistance generates heat. This means that batteries must be provided with a cooling system.

“The Toyota Hybrid System is air-cooled, and meticulous control of the cooling air enables us to keep the size of the cooling system and the capacity of the battery pack to a minimum,” Takaoka said.

Another challenge for engineers is to optimize systems by matching control techniques to battery compositions. Unlike NiMH, Li-ion is not just one battery type. Various compositions provide different traits, with different mixes in regard to power, cost, and cold-temperature operation, for example.

“A very wide range of different chemistries are being investigated. Each one has different characteristics,” said Karina Morley, Global Vice President of Control & Electronics for Ricardo.

Once automakers pick a specific Li-ion technology, most will attempt to use it throughout their product lines. Battery cells will be standardized, but other aspects of the overall battery system will generally be customized to best fit a specific vehicle.

“We won’t necessarily share battery packs across suppliers, but we can gain economies of scale with common cells,” GM’s Nitz said. "Seventy percent of battery cost is in the cell.”

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